Isolator surge protectors (ISPs) are electrical devices that are designed to block the flow of unwanted DC current in electrical systems while simultaneously allowing the flow of normal AC current to ground. ISPs also allow the flow of transient and fault currents to ground. Such fault currents may be several orders of magnitude higher than the normal operating currents of the system in which the ISP is used.
ISPs are used in various high power electrical system applications. High power transformers are used for the transmission and distribution of electrical power. These three phase transformers include a neutral line which is connected to ground. Unwanted stray DC current can flow from the ground into the transformer through this neutral to ground connection. Such transformers are not designed to accommodate a DC current flowing through the transformer windings. DC currents as low as several amperes can cause partial core saturation, resulting in excessive power losses in the transformer (i.e., excessive heating), a drop in system voltage, the introduction of undesirable harmonics, and a significant increase in noise level. Sources of DC current that can cause this problem include geomagnetically induced current (GIC) caused by solar flares, stray DC current from rapid transit systems typically found in large cities, and stray DC current associated with high-voltage DC transmission systems, particularly when operating in the monopolar mode (i.e, earth return mode). Unwanted DC current in the high power three phase transformer can be blocked by inserting an ISP between the transformer neutral connection and ground. The ISP both blocks DC current and simultaneously allows the flow of normal AC current to ground. The ISP also allows for the flow of transient fault currents to ground. Such fault currents can be several orders of magnitude higher than the normal currents found in the transformer.
ISPs may also be used in association with systems that protect metal structures against corrosion by the application of a DC bias voltage. Many metallic structures and systems are protected against corrosion by cathodic methods. For example, metallic gas transmission and distribution lines are protected against corrosion to prevent gas leaks, particularly in certain environments. Metal encased high-voltage underground transmission lines, and the metal hulls of ships, are other examples of metallic objects which are often cathodically protected.
The most common method of cathodic corrosion protection of metallic systems is to apply a negative DC potential to the system to be protected. The negative DC potential applied to the system will typically be in the 0.6 volt to 3.0 volt range relative to ground. The cathodically protected system is isolated from ground to prevent the flow of DC current from the protected system. While this procedure helps to eliminate corrosion, it introduces a potential safety problem. Often, the protected system is an inherent part of an AC power system, or is coupled to such a power system through resistive, capacitive, or inductive coupling. In the event of a fault, e.g., a short circuit, within the power system, or between the power system and the protected device, the voltage on the electrically isolated corrosion protected system may rise to unsafe levels.
To prevent such cathodically protected systems from reaching unsafe voltage levels in the event of a fault, lightning, switching transient, or other system disturbance, it is desirable that the protected system be connected to ground through an ISP or similar device. The ISP presents a high impedance to DC, at least up to the DC voltage level of interest, but presents a low impedance to AC at all times so that the voltage level on the corrosion protected system is limited to values safe for personnel and equipment.
Another application of ISPs is for the prevention of stray electrical currents associated with farm installations, particularly dairying equipment. Such stray electrical currents can present a significant economic problem for farm operations. Dairy operations are susceptible to stray electricity because cows are extremely sensitive to electricity, much more so that humans, and will respond to potentials as low as one volt or less. One solution to such a problem is to insert a blocking device, such as an ISP, between the primary and secondary neutrals of the distribution transformers serving the farm. The blocking device used opens the link between the transformer neutrals during normal operation, and closes the link very rapidly anytime the voltage between the neutrals exceeds a predetermined level. Such an overvoltage might be caused by a transformer failure, lightning surge, or other surge condition. For dairy farm applications, the blocking device will normally be required to block normal AC currents as well as DC.
Known ISPs typically include a main DC blocking (or AC bypass) capacitor which prevents the flow of DC current, while allowing the flow of normal AC currents to ground. Such ISPs preferably also include a bypass circuit, which provides a low impedance path across the capacitor when the voltage across the capacitor exceeds a predetermined level as a result of an AC or DC fault or surge condition. High speed electronic switching devices capable of handling large currents, such as silicon controlled rectifiers (SCRs), have been used to implement the low impedance bypass path. However, for applications in which the ISP is connected to systems which are subject to an external DC bias, such as cathodically protected systems, the external DC bias may be greater than the turnoff voltage of the switching devices employed in the bypass circuit. Thus, the external DC bias can operate to hold the electronic switches of the DC blocking device in a conductive state, thereby maintaining the low impedance path, and bypassing the DC blocking capacitor, even though the event which caused the triggering of the switches has ended. In such a state, the bypass path is stuck in conduction, with the external DC bias preventing proper operation of the DC blocking device.
The isolator surge protector described in U.S. Pat. No. 5,436,786, to Pelly, et al., resolves the problem of an ISP being stuck in conduction by the presence of a DC bias voltage. In accordance with Pelly, et al., an ISP preferably includes an auxiliary bypass path which short circuits the high current capacity SCRs of the main bypass path after the triggering event has passed. The auxiliary bypass path thus shunts the DC current which is maintaining the SCRs in a conducting state away from the SCRs, such that the voltage across the SCRs falls below their holding voltages. Thus, the SCRs will not become permanently stuck in conduction by the external DC bias.
In high voltage ISP applications, the energy stored in the main DC blocking capacitor at the instant that the switching devices (SCRs) in the bypass circuit are fired is relatively high. In accordance with Pelly, et al., an inductor is preferably put in series with the main DC blocking capacitor to prevent the capacitor from dumping all of its energy into the SCRs in the bypass circuit within a short time (a few tens of microseconds) after the SCRs are triggered. This energy dump may be acceptable at low voltage levels, where neither the energy stored in the capacitor, nor the instantaneous SCR voltages, are too great. At high voltage, however, the rapid dump of a large amount of energy from the capacitor could damage or destroy the bypass circuit switching devices. The inductor prevents an immediate energy dump from the capacitor. It greatly alleviates the stress on the bypass circuit SCRs by letting the capacitor dissipate its energy slowly, over a multiple number of cycles of decaying oscillations. The ISP control circuitry, which controls triggering of the SCRs in the bypass circuit, ensures that once the oscillatory discharge is set in motion (by initially firing an SCR) the SCRs in the main bypass circuit are kept in essentially continuous conduction (i.e., without the instantaneous blocking voltage applied across the bypass circuit being allowed to rise above a few volts), until the oscillation has been completed. Thus, after the bypass circuit switching devices are initially triggered at a high triggering level, e.g., 300 volts, the ISP controller establishes a period, e.g., 80 milliseconds, wherein the bypass circuit switching devices are fired at a much lower voltage level, e.g., 5 volts, until all of the energy initially stored in the DC blocking capacitor is dissipated. Without this feature, the oscillation resulting from the energy stored in the blocking capacitor and the series connected inductor would be reflected from the ISP back to the power system to which the ISP is connected, rather than being kept as an internal event within the ISP.
In previously known ISPs of the type described, continuous operation of the ISP outside of normal rated operating conditions can cause damage to the ISP's DC blocking capacitor. If the sum of a steady state external DC bias voltage applied across the ISP, plus the steady state peak AC voltage due to the AC current in the DC blocking capacitor, is higher than the trigger voltage level of the ISP, the instantaneous voltage across the capacitor will repeatedly reach the trigger level. This will cause repeated triggering, and repeated rapid high current discharge of the DC blocking capacitor through the bypass circuit SCRs. Continuous operation under such conditions can cause excessive power dissipation and possible failure of the DC blocking capacitor.
The less the DC bias voltage applied across an ISP the more AC voltage can be developed across the DC blocking capacitor, and the more AC current can flow through the capacitor, without the voltage across the ISP itself reaching the trigger voltage level. Thus, particularly at low values of DC bias voltage, the AC current through the ISP DC blocking capacitor could exceed the rms current rating of the capacitor without initiating triggering of the ISP bypass path. Continuous high AC current operation of the ISP could cause damage to the DC blocking capacitor. Previously known ISPs, which provide triggering of the bypass circuit SCRs based only on the voltage across the ISP exceeding a triggering voltage level, do not provide protection for the DC blocking capacitor by triggering of the bypass circuit SCRs in the case where the total AC current rating of the capacitor is exceeded but the trigger voltage level of the ISP is not reached.